Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. The former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art FIG. 1 (adapted from U.S. Pat. No. 7,163,278) as an example of a conventional thermal inkjet drop-on-demand drop ejector configuration. Partition walls 20 are formed on a base plate 10 and define pressure chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32 (also called orifices herein), each nozzle 32 being disposed over a corresponding pressure chamber 22. Ink enters pressure chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink inlets 24, as indicated by the arrows in FIG. 1. A heating element 35, which functions as the actuator, is formed on the surface of the base plate 10 within each pressure chamber 22. Heating element 35 is configured to selectively pressurize the pressure chamber 22 by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle 32 when an energizing pulse of appropriate amplitude and duration is provided.
Drop ejector array devices for inkjet printers, whether for pagewidth printers or for carriage printers, typically have hundreds of drop ejectors that are connected to a power bus that extends along the length of the drop ejector array. The power bus is connected at one or both ends to a voltage source. The drop ejectors are also connected (either directly or indirectly through driver transistors) to a common current return bus, which is typically connected at one or both ends to ground. Firing all drop ejectors in the array at the same time would require excessive instantaneous current. Firing only one drop ejector at a time would result in slow printing speeds. Typically, groupings of drop ejectors are enabled sequentially to print one grouping at a time. Each grouping requires only a short firing time and then has a rest period while the other groupings are sequentially fired before it is time to fire the grouping again. During the rest period, ink refills the pressure chambers of the ejectors that have been fired and resumes a sufficiently stable state for uniform drop ejection performance.
The power bus and the current return bus are made of an electrically conductive material such as aluminum. However, they are typically on the order of one to two microns thick. As a result, their resistance can be several ohms, which is not an insignificant fraction of the resistance of the heating element 35. The resistance in the bus lines is sometimes called parasitic resistance. When one or more actuators are fired, the current through the bus lines results in parasitic voltage drops. The amount of parasitic resistance between a given actuator and its connections to power and ground depend upon the position of the actuator in the array. The parasitic voltage drop for actuators of drop ejectors that are near an end of bus lines that are connected to power and ground is less than the parasitic voltage drop for actuators of drop ejectors that are farther from power and ground connections. In addition, the amount of current flowing through the bus lines (and therefore the magnitude of the parasitic voltage drop) depends upon how many drop ejectors are fired simultaneously. Print data can sometimes require one drop ejector in a grouping to be fired, or multiple drop ejectors in the grouping, or even the entire grouping at one time. The parasitic voltage drop due to bus line resistance thus depends upon the number of actuators that are fired as well as the location of the actuator or actuators along the drop ejector array.
Reliable drop ejection in a thermal inkjet printhead requires that the ink in each drop ejector to be fired is brought to rapid boiling in order to nucleate a vapor bubble that grows and expels the drop, regardless of the location of the drop ejectors or the number fired simultaneously. If the voltage provided to the power bus line is too small, only the drop ejectors that have smallest parasitic voltage drop will fire reliably. A threshold voltage can be defined as the voltage at which a drop ejector with the smallest parasitic voltage drop will reliably eject drops when fired without other drop ejectors firing. Typically the voltage (called an overvoltage) that is provided to the power bus for printing is somewhat greater than the threshold voltage. Excessive overvoltage can have adverse effects including increased power dissipation, drop nonuniformity, and damage to the heating elements. Drop ejectors at locations that are closer to the power and ground connections are more susceptible to excessive overvoltage, especially when fired one at a time, due to the lower parasitic voltage drop.
A variety of device designs have been disclosed in the prior art for compensating for variations in parasitic voltage drops in order to reduce the amount of overvoltage that is required to ensure that all drop ejectors can fire, even when multiple drop ejectors are fired at the same time. U.S. Pat. No. 4,887,098 discloses providing two common bus lines that are connected together with lines that pass between adjacent heating elements. The first common bus line and the second common bus line extend along opposite ends of the heating elements. Connection of the heating elements to the driver transistors is made by lines that cross over or under the second common bus line. U.S. Pat. No. 5,144,341 is similar to '098 but also includes a series ballast resistor disposed between the first power bus and the voltage input that the first power bus is connected to. The device design disclosed in U.S. Pat. No. 6,398,347 to compensate for the variation in parasitic resistances in the power bus lines uses configuring of the driver transistors to have differing on-resistances. For example, the on-resistance of the FET drivers is individually configured by the length of a continuously non-contacted segment of the drain region fingers. Alternatively, the on-resistance can be configured by varying the area of the FET driver transistors.
Other ways to compensate for variations in parasitic voltage drops disclosed in the prior art have included methods of operating the device. U.S. Pat. No. 5,497,174 discloses adjusting the duration of the pulse applied to the heating elements. For small parasitic voltage drops a shorter pulse duration is used, and for larger parasitic voltage drops a longer pulse duration is used. In this way the total energy provided to the heating elements is made more uniform. U.S. Pat. No. 5,469,203 discloses a similar approach using pulse count variation for compensating for parasitic voltage drops in a thermal printhead (not inkjet). U.S. Pat. No. 6,976,752 discloses associating a compensation circuit with each drop ejector where each compensation circuit includes a plurality of switches connected in parallel with each other. Internal resistance of the compensation circuits is adjusted by turning on more or fewer switches and thereby compensates for variations in parasitic resistance of the power lines depending, for example, on the number of drop ejectors to be fired at one time. U.S. Pat. No. 8,757,778 discloses using electronic circuitry to compensate for variations in parasitic voltage drops by monitoring ground potential and other supply-related voltages, and providing signals to affect the biasing of one or more transistors that couple the heating elements to supply voltage or ground.
Some of the ways of compensating for variations in parasitic voltage drops disclosed in the prior art are best suited for printheads where the drop ejectors to be fired at one time are proximate to one another on the printhead. For example, both '098 and '341 contemplate firing groups of adjacent drop ejectors at the same time. Compensation is provided such that, for firing groupings of four adjacent drop ejectors for example, the parasitic voltage drop for groupings near the middle of the array is made to be more similar to the parasitic voltage drop for groupings near the ends of the array. However, firing groupings of adjacent drop ejectors at the same time can result in undesirable fluidic cross-talk that affects drop ejection performance, and can require a longer rest time before the grouping of drop ejectors can be fired again. Other ways of compensating for variations in parasitic voltage drops can require additional area for implementation, thereby increasing the size and the cost of the drop ejector array device. For example, '347 requires varying the size of the driver transistors, and '752 requires the addition of many additional switches to the drop ejector array device. In addition, on-resistance of FET drivers, as disclosed in '347, can be difficult to control with sufficient accuracy. Similarly, ways of compensating for variations in parasitic voltage drops that rely on biasing of transistors, as disclosed in '778, can also be difficult to control with sufficient accuracy. Ways of compensating for variations in parasitic voltage drops based on varying the pulse count or pulse duration, as disclosed in '203 and '174, can result in a longer time to fire all of the drop ejectors, thereby limiting print speed. Ways of compensating for variations in parasitic voltage drops based on turning on more or fewer switches, as disclosed in '752, can result not only in a larger and higher cost drop ejector array device as noted above, but also can require increased data processing time.
Despite the previous advances in compensating for variations in parasitic voltage drops, what is still needed are drop ejector array device configurations and methods of operation that are effective in compensating for variations in parasitic voltage drops when the drop ejectors fired simultaneously are more widely spaced apart rather than being adjacent to one another. Furthermore, it is desired that such drop ejector array devices be compact, compatible with high speed printing with good drop ejection uniformity and long actuator lifetime, and not require additional input/output terminals on the drop ejector array device.